High voltage field effect transistors are known. A limit on the performance of such devices is the maximum voltage across the device that they withstand before breakdown mechanisms cause the current flow in the device to increase in an uncontrolled manner. This leads to loss of control of the device, and often to device failure due to ohmic heating of the device. One known way to increase the breakdown voltage is to use “field plates” to allow depletion regions to extend over greater distances with the device, thereby reducing local electric field strengths. However devices employing this technique may not last as long as expected.